Boilers are used for burning burnable material for producing energy. An example of a boiler 100 of prior art is shown in a side view in FIG. 1a. The boiler comprises a furnace 110, in which the burnable material is burnt, as well as a duct 112 for supplying combustion air 150. Heat is produced in a combustion process in the furnace, which heat is recovered from flue gases and radiation. Heat can be recovered, for example, by superheaters 120 at the top of the furnace. From the furnace, the flue gases are led into a flue gas duct. From the flue gas duct, heat can be recovered by evaporators 130. The superheaters 120 and the evaporators 130 typically comprise heat exchanger pipes 122 and 132, respectively, for recovering heat. Heat can be recovered in, for example, water or steam. Furthermore, water can be preheated in a water preheating area 200. Moreover, heat can be recovered in combustion air 150 in an air preheating area 250. Cooled flue gases 160 can be removed from the process. For example, supporting structures for water and air preheaters can be provided in a smoke passage 300 between the water preheating area 200 and the air preheating area 250. The figure also shows the front wall 302 and the rear wall 304 of the flue gas duct. The flue gas duct may have a substantially rectangular shape in the cross-sectional plane 301 transverse to the flow direction of the flue gases, wherein it may comprise side walls which are substantially transverse to the front wall 302 and parallel to the direction of the flow of the flue gas. In a way corresponding to the smoke passage, the water preheating area 200 may also comprise a front wall 202 and a rear wall 204. In a corresponding way, the air preheating area 250 may comprise a front wall 252 and a rear wall 254. FIG. 1b illustrates the mutual location of the front wall 202, the rear wall 202, the first side wall 206, and the second side wall 208.
A problem in boilers of prior art is the noise produced in them. The noise produced typically has a frequency between about 40 Hz and about 100 Hz. The noise is typically tonal; that is, the noise is loudest close to a given frequency or close to some given frequencies. This given frequency may be influenced by, for example, the dimensions of the boiler or the flue gas duct. The tonal noise differs from, for example, noise in which the vibration takes place at substantially the same volume in a wide frequency band.
FIG. 2a shows a more detailed side view of the water preheating area 200. One possible source of the noise is turbulence of the flue gas flow. Turbulence is caused by, for example, the water preheaters 210 in the water preheating area 200. The preheaters 210 may be, for example, pipe bundles. FIG. 2b illustrates the pipe bundles in more detail in another side view, substantially transverse to FIG. 2a. The pipe bundles comprise heat exchanger pipes 212. According to FIGS. 2a and 2b, the pipes 212 can extend from the front wall 202 to the rear wall 204. The turbulence can also be influenced by the air preheaters 260 in the air preheating area 250.
FIGS. 2c and 2d illustrate the air preheaters 260 which can also be pipe bundles and comprise pipes 262. The flow of combustion air 150 in the air preheater is shown with arrows in FIG. 2c. Also, the air preheating pipes 262 can extend from the front wall 252 to the rear wall 254 of the air preheating area 250. The air preheating pipes 262 are located between the side walls 256 and 258 of the air preheating area 250.
According to prior art, noise can be prevented in boilers by plates provided in the flue gas duct. The plates can be, for example, metal plates intended for affecting the formation of a standing sound wave and its wavelength. However, sufficient noise suppression has not been achieved in the prior art. In some solutions, the placement of the plates in the flue gas duct makes it more difficult to remove dust from the flue gas duct.